Gram-negative bacterial lipopolysaccharide (LPS) is one of the best characterized triggers of "macrophage activation" leading to the production of inflammatory mediators including tumor necrosis factor-alpha (TNF) and nitric oxide (NO). The cytokine interferon-gamma (IFN-gamma) primes macrophages leading to an augmented response to stimuli such as LPS. Several investigators have shown that tyrosine phosphorylation is essential for macrophage activation by rlFN-gamma and LPS, but the identity of the kinases and targets of phosphorylation that mediate this process remains uncertain. The recent development of novel, reportedly specific inhibitors of tyrosine kinase and MAP kinase pathways provides investigators with the opportunity to carefully dissect the signaling pathways responsible for macrophage activation. This will be carried out in the proposed research by addressing the following three specific aims: (1) determine the ability of selective tyrosine kinase inhibitors and other specific kinase inhibitors to block macrophage activation (2) compare and contrast the signaling pathways necessary for induction of TNF versus NO by murine macrophages, and (3) identify specific substrates of tyrosine phosphorylation during macrophage activation and determine which substrates are necessary/sufficient for the production of key inflammatory mediators.